In sports racquets having composite frames, the ends of the strings forming the string bed are secured in holes formed in the racquet frame walls. The strings, after crossing the string bed, pass through one string hole, are directed along the outside of the frame to the next string hole, and thereafter reenter the string bed area through the second string hole.
At the points where the strings enter and exit the string holes on the outside of the frame, the strings bend sharply. In order to prevent the composite material from damaging the strings, or vice-versa, it is customary to provide plastic grommet strips along the sides of the frame, and a plastic bumper strip in the tip region of the racquet. The grommet sleeves and bumper strip both have a strip portion that extends along the outside of the frame, and a plurality of hollow grommet pegs that extend through the string holes in the frame. In this manner, the strings, when passing through the string holes and along the outside of the racquet, contact the plastic grommet strip or bumper strip rather than the composite frame.
The grommet strips are normally disposed in a stringing groove formed in the outside surface of the frame. The bumper strip includes a center portion, which is the same as the grommet strip and that is disposed in the stringing groove, but also includes a pair of wings that project laterally from the center portion to overlie the outwardly facing surfaces of the frame, and thereby protect the frame surface from impact damage from impact, e.g., with the ground.
A racquet's power and sweet spot size are related to the length of the string used. Thus, an oversized racquet, such as disclosed in Head U.S. Pat. No. 3,999,756, increases the racquet's power and sweet spot size, because the strings are longer than in prior small head racquets, deflect more at impact, and reduce the compression of the ball at impact (which reduces energy loss). If the energy lost at ball impact can be reduced or conserved, more energy can then be returned to the ball via the strings and more power results from the hit.
While increasing a racquet's head size increases power, many players feel that racquets with large head sizes have less maneuverability, since the larger head size results in a larger polar moment of inertia. Although a larger polar moment of inertia is desirable in certain respects, because it increases the size of the sweet spot and reduces the tendency of the racquet to twist should the ball land other than along the centerline of the string bed, thereby making the racquet more stable, some players feel that it makes it harder to pronate the racquet for making effective spin shots and serves. It therefore would be desirable to increase the effective length of the strings without increasing the size of the head.
When a ball impacts the string bed, the strings are deflected in a direction perpendicular to the string bed. The opposed ends of each string bend about the grommet pegs, i.e., at the point where they exit the grommet peg and enter the string bed, which is adjacent the inside wall of the frame. Therefore, the effective string length is the distance between opposed walls at the location where each string spans the string bed.
Soong U.S. Pat. No. 5,014,987 discloses a racquet in which the string holes on the inside wall of the frame are greatly enlarged, so that the string can deflect freely. In such manner, the strings are secured only in the outside wall of the frame, and the effective string length is the distance between opposed outside walls, not the distance between the inside walls. Klose et al. U.S. Pat. No. 5,332,213 employs the same concept of enlarging the string hole on the inside wall of the frame to allow the string to deflect freely. However, instead of eliminating the grommet peg on the frame's inside wall, Klose et al. propose increasing the height of the grommet hole towards the inside of the frame.
As strings become longer and therefore more resilient, they tend to vibrate longer, and with a greater amplitude, after the ball leaves the string bed. String vibration is annoying to players, and therefore a number of methods have been proposed to damp string vibration. For example, Davis U.S. Pat. No. 4,828,259 discloses a tennis racquet with a double bridge in the throat region, that contains a vibration-dampening bar that damps both string vibrations and racquet frame vibrations. Davis et al. U.S. Pat. No. 5,211,397 discloses a vibration dampening member that engages the string bed for damping vibration. However, it is generally more preferable where possible to have a built-in vibration damping system such as in the '259 patent.